i. Field of the Invention
This invention relates to compositions and procedures for the treatment of wood and wood products. Such compositions and treatment procedures are designed to provide protection not only during the storage and the handling of the lumber, or as use as a primer of sheathing, but also to provide protection during the final use of the wood and wood products. It also relates to the treated wood products so formed.
A piece of timber, due to the manner of its formation, possesses anisotropic structure which influences its properties and behaviour. Compared to competitive cladding materials, for example, metals and plastics materials, it has a number of major disadvantages which tend to counteract the advantages of strength, lightness, low thermal expansion and, in some instances, desirable aesthetic features. To overcome and minimize these disadvantages a number of specific problems exist: the wood must be protected against degrading environmental factors (namely, moisture cycling, photodegradation and biological attack); the dimensional stability with respect to moisture cycling must be improved; photodegradation due to sunlight must be minimized; the resistance to biological attack (fungus) must be improved; the adhesion of protective and decorative coatings must be improved; and extractives which adversely affect protective and decorative properties of coatings must be sealed within the wood. For some purposes, such aims should be achieved with a treatment that does not alter the natural beauty of wood.
II. Description of the Prior Art
Protective systems should impart not only protection against biological degradation, but also considerable weather resistance. The special needs for some purposes are high protection against weather, and against biodegradation and, additionally, providing protection against fire and maintaining the natural appearance of wood without discoloration after treatment. The system should provide weather resistance with enhanced glowing combustion resistance and no discoloration after treatment. The treated material should also be clean and paintable.
Four classes of treatments are currently used in an attempt to meet these requirements.
The first class is that of clear or pigmented penetrating systems which contain fungicides and water-repellent additives, such as, for example, polyethylene waxes and metal stearates in a non-aqueous solvent media. These treatments are deficient in that they must be repeated at regular intervals of 1 to 2 years to provide a desired level of protection.
The second class is that of stains and sealers. These are normally synthetic resin solutions, usually pigmented and designed to penetrate the surface of the wood. These treatments as well are deficient, and should be repeated every 1 to 2 years in order to provide the required degree of protection.
The third class is that of paint system. Such paint systems would normally consist of a primer and top coats. When well applied, these will provide the exterior cladding protection from 2 to 5 years.
The fourth class is that of salt treatments. A number of salt treatments have been suggested, the most common of which are known as copper-chrome arsenate (CCA), zinc metaarsenate (ZMA), acid-copper-chromate (ACC), and ammoniacal copper arsenite (ACA). Present systems of this type are effective to provide relatively long term durability when applied by pressure impregnation techniques. While such systems are effective in preventing biological deterioration of wood and provide clean and paintable surfaces, they, however, suffer from several disadvantages. CCA preservatives have low stability under storage and processing conditions and penetration into wood is limited. ACC preservatives cannot penetrate refractory species but water repellency of wood treated by ACC is good. Ammoniacal copper salts provide extremely stable treating solutions and produce a treated wood product which is deeply penetrated (particularly in the case of refractory species), but which provides negligible protection to the products against weathering. All systems provide products which are more or less colored; thus the natural appearance of wood is changed or covered by the colored preservative. Furthermore, none of these systems provides protection against glowing combustion. The CCA systems are believed to become fixed in the wood by oxidation-reduction reaction associated with the chromic acid in the compositions and it is these same reactions which are believed adversely to affect stability and processing characteristics. Furthermore, the arsenic-containing formulations mostly have little leach resistance, resulting in environmental pollution by leached arsenic.
A paper in the Forest Products Journal, Vol. 22, No. 11, November 1970 by M. P. Levi et al. entitled "Distribution and Effectiveness in Pinus Sp. of a Water Repellent Additive for Water-Borne Wood Preservatives" discusses the development of a water repellent additive for use with water-borne CCA preservatives to overcome the deficiency of prevention of weathering degradation.
Ammoniacal copper arsenite compositions are presently being used as preservatives. Zinc arsenate, zinc arsenite, and zinc phosphate can all be applied from an acetic acid solution and, on drying, the salt is water insoluble but very poorly fixed in the wood. However, in all of these cases, the weather resistance of the treated wood is not significantly improved.
Thus, each of the preserving systems mentioned above has disadvantages and these can be summarized as follows.
The major disadvantage of the use of copper-chrome-arsenate systems is limited stability of the treating solution under processing conditions, low penetration into difficult-to-penetrate species, and relatively low leach resistance of the arsenic.
The major disadvantage of the use of ammoniacal copper compounds is the lack of weathering resistance of the treated wood product, and very poor fixation and very low leach resistance of arsenic in treated wood.
Copper and zinc-containing fungicides which have been proposed, (see U.S. Pat. No. 2,414,661 issued Jan. 21, 1947 to A. A. Nikitin), were prepared by precipitation of a zinc salt and a copper salt from an aqueous solution with an alkali solution containing soya bean protein, or soaps of fatty acids.
Fungicides, which have been proposed for cellulosic materials, (see U.S. Pat. No. 2,423,619 issued July 8, 1947 to L. Roon), comprise copper soaps formed in situ from an aqueous solution of copper salts and aqueous ammonia by reaction with fatty acids.
It has also been proposed to provide water and fire-resistant coatings on wood, (see U.S. Pat. No. 2,530,458 issued Nov. 21, 1950 to H. R. Frisch) by the use of zinc orthophosphate or zinc orthoarsenate compositions applied as a concentrated solution in aqueous ammonia.
It has been proposed, (see U.S. Pat. No. 2,768,910 issued Oct. 30, 1956 to H. Krzikalla and O. Lissner) to improve the hardness, compressive strength, hygroscopicity and liability to swell of wood by impregnating the wood with an aqueous ammoniacal solution of polycarboxylic acid containing at least six carbon atoms.
It has been proposed, (see U.S. Pat. No. 2,772,263 issued Nov. 27, 1956 to C. C. Yeager) to use a compound having a high fungicidal activity in wood, which compound is a metal rosin ammonium phenoxide-complex metal carboxylic acid soap compound, prepared by reacting a rosin ammonium phenoxide with a water-soluble salt of a metal capable of forming a complex with ammonia.
It has also been proposed, (see U.S. Pat. No. 3,007,844 issued Nov. 7, 1961 to W. O. Schuly) to use a composition comprising a heavy metal ion, borate ions and chromate ions as an impregnating agent for the preservation of wood.
It has further been proposed, (see U.S. Pat. No. 3,105,773 issued Oct. 1, 1963 to S. Frank and D. C. Wehner) to preserve wood by imparting pesticidal and anti-thallophytic properties thereto by first impregnating the wood with a water-soluble heavy metal salt, and then with an acrylic polymer solution.
It has still further been proposed, (see U.S. Pat. No. 1,942,977 issued Jan. 9, 1934 to E. E. M. Payne) to treat wood products with a solution of one or more ammonium phosphates and then with a solution containing acid phosphates of magnesium and zinc, in order to precipitate an insoluble phosphate within the cell structure of the material, thereby to improve the color of the materials and to render the treated material resistant to fire.
It has also been proposed, [see British Pat. No. 1,220,281 published Jan. 27, 1971 in the name of Hickson's Timber Impregnation Co. (G.B.) Ltd.] to treat wood with an aqueous emulsion containing an aqueous solution of a wood preservative composition based on hexavalent chromium, a water-insoluble insecticide in a liquid hydrocarbon solvent, and a non-ionic surface active agent. The emulsion is used by impregnation of the wood by means of a pressure process, to provide protection against fungal attack and against a variety of insects.
Finally, it has been proposed, (see Canadian Pat. No. 568,393 issued Jan. 6, 1959 to B. O. Hager) to provide a wood preservative composition consisting of an ammoniacal aqueous solution containing an amine-forming metal, e.g. Cu or Cu and Zn, with or without arsenic and carbon dioxide of a content at least two-thirds of the metal content. The ratio of CO.sub.2 /NH.sub.3 /Cu and/or Zn/As is 1.8-5.5/3.2-4.8/1.6-2.4/0.9-0.96.
While the use of the compositions outlined above has tended to provide a considerable level of protection against specific degrading agencies, none of them provides a suitable balance of properties such as, for example, excellent stability under processing conditions, high weather resistance, paintability, good wood penetration and good water repellency, and low arsenic leachability. Additionally, some of the compositions outlined provide in one system some measure of protection against fire and do not adversely affect the natural appearance of the treated wood.